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| 型號(hào)： | LM2795 |
| 文件頁數(shù)： | 11/14頁 |
| 文件大小： | 373K |
| 代理商： | LM2795 |
Application Information
(Continued)
LED SELECTION
The LM2794/5 are designed to drive LEDs with a forward
voltage of about 3.0V to 3.8V. The typical and maximum V
F
depends highly on the manufacturer and the technology.
Table 2
lists two suggested manufactures and example part
numbers. Each supplier makes many LEDs that work well
with the LM2794/5. The LEDs suggested below are in a
surface mount package and TOPLED or SIDEVIEW configu-
ration with a maximum forward current of 20mA. These
diodes also come in SIDELED or SIDEVIEW configuration
and various chromaticity groups. For applications that de-
mand color and brightness matching, care must be taken to
select LEDs from the same chromaticity group. Forward
current matching is assured over the LED process variations
due to the constant current output of the LM2794/5. For best
fit selection for an application, consult the manufacturer for
detailed information.
TABLE 2. White LED Selection:
Component
LWT673/LWT67C
NSCW100/NSCW215 Nichia
Manufacture Contact
Osram
www.osram-os.com
www.nichia.com
I
SET
PIN
An external resistor, R
, sets the mirror current that is
required to provide a constant current through the LEDs. The
current through R
and the LED is set by the internal
current mirror circuitry with a ratio of 10:1 The currents
through each LED are matched within 0.5%. R
SET
should be
chosen not to exceed the maximum current delivery capa-
bility of the device.
Table 3
shows a list of R
values when
maximun BRGT = 3V is applied. For other BRGT voltages,
R
SET
can be calculated using this equation:
R
SET
= ((0.188 + (0.385
BRGT)) / I
SET
)
10
TABLE 3. R
SET
Selections ( when BRGT pin = 3V
maximum)
I
LED
per LED
15mA
10mA
5mA
*R
SET
(+/-1%)
909
1.4K
2.67K
* The Rset values are rounded off to the nearest 1% standard resistors
BRGT PIN
The BRGT pin can be used to smoothly vary the brightness
of the White LEDs. In the LM2794/5, voltage on BRGT is
connected to an internal resistor divider which gives a factor
of 0.385 and summed with an offset voltage (188mV) from
the bandgap (See Functional Block Diagram). This voltage is
fed to the operational amplifier that controls the current
through the mirror resistor R
SET
. The nominal range on
BRGT is 0V to 3V.
Care must be taken to prevent voltages on BRGT that cause
LED current to exceed a total of 80 mA. Although this will not
cause damage to the IC, it will not meet the guaranteed
specifications listed in the Electrical Characteristics.
Table 4
shows the current through each LED for the LM2794/5 with
various BRGT and R
SET
values using I
LED
equation below.
TABLE 4. LED Current When Using BRGT Input
(Values Highlighted in Boldface exceeded maximum current range of the device)
R
SET
(
)
V
BRGT
(V)
0.0
0.5
1
1.5
2
2.5
3
124
I
LED
(mA)
15.16
30.69
46.21
61.73
77.26
92.78
108.31
500
I
LED
(mA)
3.76
7.61
11.46
15.31
19.16
23.01
26.86
900
I
LED
(mA)
2.09
4.23
6.37
8.51
10.64
12.78
14.92
1750
I
LED
(mA)
1.07
2.17
3.27
4.37
5.47
6.57
7.67
Calculation of LED Current When Using BRGT :
V
IN
= 3.6V
R
SET
= 1000
, BRGT = 3V
I
LED
= ((V
OFFSET
+ (0.385
BRGT))/1000 )
10
I
LED
= ((0.188 + (0.385
3)) / 1000 )
10 = 13.4mA
BRIGHTNESS CONTROL USING PWM
Brightness control can be implemented by pulsing a signal at
the SD pin. The recommended frequency is between 100Hz
to 1kHz. If the PWM frequency is much less than 100Hz,
flicker may be seen in the LEDs. Likewise, if frequency is
much higher, brightness in the LEDs will not be linear. When
a PWM signal is used to drive the SD pin of the LM2794/5,
connect BRGT pin to GND. The R
value is then selected
using the above I
equation when BRGT = 0V. The
brightness is controlled by increasing and decreasing the
duty cycle of the PWM signal. Zero duty cycle will turn off the
LED and a 50% duty cycle waveform produces an average
current of 7.5mA if R
is set to produce a maximum LED
current of 15mA. So the LED current varies linearly with the
duty cycle.
P
OUT
The charge-pump output voltage (P
) on the LM2794/5
can be used to deliver additional current to other circuitry if
desired. The available current from P
depends on the
total LED current consumed and the diode forward voltage.
The graph on page 7 (typical performance) shows the addi-
tional available output current from P
when all four di-
odes are consuming a total current of 60mA. The graph
shows that the available additional current from P
will
vary with diode forward voltage. Moreover, if the total diode
current is reduced below 60mA, then more current will be
available from P
. It is imperative not to exceed the maxi-
mum power dissipation of the device when P
OUT
is used to
L
www.national.com
11
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